This research continues our long-term interest in the central problem of reproductive immunology and genetics: how the semiallogeneic fetus avoids rejection. The specific aims are: First, the Pa, 213 and A/a antigens will be localized on the placenta. The morphological approach will utilize staining with the biotinylated Fab fragments of the respective antibodies and cold antibody inhibition studies to demonstrate specificity. In vivo placental immunoabsorption will be studied using radiolabeled antibodies. Cells labeled intrinsically with radioactive amino acids will be labeled extrinsically with iodine to identify the antigens that are made in the cell but not expressed on its surface and those that are expressed. Second, the structure of the class I antigens Pa, 213, A/a and F/a isolated from trophoblast cells and from lymphocytes will be compared by SDS- PAGE, isoelectric focusing, sequential immunoprecipitation, peptide mapping and carbohydrate analysis. Third, the genetic control of the expression of antigen 213 will be studied in the appropriate crosses among inbred and congenic strains. Finally, a molecular analysis of the Pa, 213 and A/a antigens in the placenta will be undertaken. The mRNA will be isolated from the placenta, and cDNA will be prepared in a lambda expression vector. The clones will be screened with murine class I probes, the appropriate cDNA will be isolated, mapped and subcloned into stable plasmids. These subclones will be sequenced and their products characterized by Western blotting. They will then be used for quantitative assessment of the three antigen transcripts in the placenta. Investigation of why the fetal allograft is not rejected will provide insight into the immunogenetics of an apparently anomalous transplantation situation and into the pathogenesis of chronic spontaneous abortion and developmental anomalies. It may also provide insight into the mechanism by which tumors avoid host immunosurveillance. This information could be used to design radically new methods of suppressing organ allograft rejection and of enhancing tumor rejection.